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You said it well, Ernest
What you want is heat removal from the engine. Slowing flow down
through a radiator will indeed show a larger Delta T from intake to outlet
because the coolant(in the radiator) is exposed to cooling air longer.
That has led many to believe erroneously that slow flow = more heat
removal. I once argued an hour with old man Lou Ross about this issue and
when I told him that the obvious conclusion was that if slowed water cooled
better, then stopped water would cool best – there was silence on the other end
of the phone and then Lou hung up and never spoke to me again.
Part of the myth also stems the results when attempts were made to improve
flow rate by speeding up the coolant pump expecting better cooling. When
worst cooling occurred, it was concluded erroneously that the faster flow
resulted in worst cooling. In most if not all of those instances, the poor
cooling resulted from less flow – the faster water pump was actually cavitating
and therefore actually pumping less coolant than at slower rotating speeds where
cavitation did not occur. But, it all fed into the myth.
Heat duty (Q): Heat duty is defined as the product of mass flow
rate specific heat capacity and the temperature difference
between inlet and outlet fluid temperatures
Q = m*cp* DT
A rule of thumb regarding heat removal and flow rate is:
The heat transfer coefficient
decreases by ˜10% with a threefold increase in the mass flow rate
So a 10% decrease in transfer resulting from three times the mass
flow shows that increased mass flow (in of itself) will result in increased heat
removal even though the heat transfer rate may lessen slightly.
At some point you get pressure losses and other factors - not to mention
the greatly increased power required to get the large increase in mass flow -
makes it impractical to infinitely increase flow rate. Once you get the
flow good enough to cool your engine under whatever the conditions you are
operating within, it makes little sense to waste power to get more flow
However, we want best heat removal from the engine. Heat is removed
via mass flow of the liquid – no mass flow = no cooling. So the more mass
flow(within reason) - the more heat is removed from the engine and provided we
can get rid of a certain amount of that heat though the radiator the more
cooling of the engine occurs. So it’s a system, the cooler the oil returned to
the engine the better heat transfer, the more mass flow from engine to radiator
the more cooling, the cooler the air flowing across the radiator the better the
heat rejection, etc. All factors contribute and you can not focus on just
one factor, the optimum cooling results from the optimization of all the major
variables involved for a particular situation.
Just my 0.02 Back to my cave
Ed
.
Sent: Sunday, June 24, 2018 3:41 PM
Subject: [FlyRotary] Re: Oil
Isn't the opposite also true. The most efficient removal of
heat from the back side of the rotor will occur when the oil is the
coolest?
I suspect that the actual system efficiency curve is very
flat.
On Sunday, June 24, 2018 2:08 PM, "Charlie
England ceengland7@gmail.com" <flyrotary@lancaironline.net>
wrote:
First, let me say that I'm far from
being an authority on this subject. The idea of coolant (oil,
water, air, etc) moving too quickly through a heat exchanger comes up often.
People who's opinion I trust (trained engineers) say that slowing flow does not
improve efficiency. What I've been told is that yes, you may see higher delta T
across the cooler with lower flow, but that's not a true and complete picture of
what's happening. My understanding, based on what I've read & been told, is
that the best heat exchange occurs with the max temperature difference between
the media (oil>air, water>air, etc). If you slow the flow through the
exchanger, then yes, you will see a bigger delta T across the exchanger, but
that means that a lot of the oil (in this case) in the exchanger has already
been cooled 'early' in the flow, so effectively, part of the exchanger is
operating at a much lower temperature difference with the air, and therefore,
its efficiency is reduced. So it follows that higher flow, keeping the entire
exchanger hotter (lower delta T) actually improves efficiency. Yes, it's
counter-intuitive (at least for me). But supposedly, the most BTUs get removed
from the system when the entire exchanger is kept at close to the same temp
across its face. There's obviously a point of diminishing returns, where
you're actually adding heat by overpressurizing the flow path trying to speed up
flow, but I doubt we're there yet. :-) Perhaps a real engineer could step
in and clarify. On 6/23/2018 9:35 PM, Andrew Martin andrew@martinag.com.au
wrote:
Lynn, my setup is pretty much stock where most oil should pass through
cooler direct to rear iron ocv, only oil that enters oil gallery is filtered,
pressure, temp & redrive oil taken from a block after filter,
But the cooler issue is a bit more incidious in that without a pressure
gauge at pump outlet there is no indication of the restriction. I have no
problem with having “some” restriction in the cooler but as it builds markedly
with increased flow at rpm, Oil delta t drops as oil flow is too fast through
the core to cool the oil, and when front cover relief opens at high rpm due to
the restriction, only part of pump output is getting cooled and temps rise
more.
Setrab, Fluidyne etc do claim low pressure drop but I have struggled to
find at what flow rates, Adding smaller coolers in parallel is an option but
the data is still needed to choose the correct sizes that allows all oil to
pass through a cooler without pressure drop and have just enough surface area
to transfer heat to air.
My test showed 140psi pump output 80psi at back iron, I still dont know
what my front cover relief is set at, as 140 was max pressure of gauge I had.
But front cover relief valve should never operate in normal operation as it is
a safety valve for the pump,front cover & cooler only.
Only engine that is diferent is 2009+ renesis as that has only one valve
in the system & diferent oil flow design to the rest of the mazda
rotaries.
Andrew
A restrictive cooler would (might) show a higher oil pressure than the
control valve will allow if measured before the cooler. Because the stock
relief valve is at the end of the system. So the stock valve might
allow for 80 PSI, but never open if the full 80 PSI never gets to it so as
to activate. Racers measure oil pressure where the oil enters the engine.
Usually in an aluminum block that replaces the stock oil filter
stand.
What
do the bearings see, is the information you want. We raced for years with 80
PSI entering the engine.
And
that was turning the engine to 9,000 RPM on each shift. Oil coolers are
constructed of many sharp edged tubes . Pushing oil or any liquid or gas
into the end of a sharp edged tube is nearly impossible. So many more tubes
than you would calculate necessary are used in order to overcome the sharp
tube flow problem. So,
if the stock relief were set at 79 PSI (stock on early engines) you
would want to see 79 PSI on you oil pressure Gage as taken out of that
aluminum block. Mistral calculated the cooler size required on the test
Piper. The plane would overheat the oil while still within sight of the
airport.
The
were also using aircraft oil in the engine. 20-50 if I remember correctly.
So, flow got worse as the oil heated up.
The
racer had an external oil pump with one pressure section (adjustable up to
any pressure you might want) and two scavenge sections. The scavenge
sections returned oil and air to a storage tank through a set of bug screen
filters and two Setrab 44 row coolers in series. The pressure section pulled
from the tank and pressurized oil went through two K&N oil filters in
parallel and then through a single 44 row Setrab cooler. So, we ran 100 PSI
at the engine. Shifting at 9,700 RPM. 250 HP. Oil is Red Line 20-50
racing synthetic. A common choice for rotary racing. Not a single oil
related failure in 30 years. Oil coolers (and filters) in parallel reduce
flow resistance. Coolers and filters in series increase flow resistance.
Racing oils collect heat and give it up more quickly than do conventional
oils.So any cooler performs a bit better with a synthetic.
Lynn
E. Hanover
Any
question, any time.
In a message dated 6/23/2018 4:59:30 AM Eastern Standard Time, flyrotary@lancaironline.net
writes:
Just got around to plumbing in mechanical gauge before cooler to see
whats really happening with my oil flows, wish I’d done it years ago!
Learnt so much in a couple of minutes on things that I have wasted so much
time second guessing. my second attempt oil cooler did work better than
the original mazda cooler, but was atrocious overall, Pressure drop was
about 60psi at 1400 prop rpm. No wonder I cant cool the oil, bugger all is
going through it, just enough to give me about 80psi oil pressure.
Ended up bypassing cooler all together to confirm it is the cooler
that is problem not lines or anything else, well what a diference
pressures constant at 78psi at all rpm’s
Trouble is no cooler manufacturer here seems to have charts of flow
& pressure drop on their coolers, very frustrating especially as
prices seem to range between $100-900 for similar sizes, so makes it very
hard to select correct one.
Andrew --
Regards Andrew Martin Martin
Ag --
Regards Andrew Martin Martin
Ag
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